Lookalike finally suffering unwelcome stabbing pain (6)
I believe the answer is:
twinge
'stabbing pain' is the definition.
(thesaurus)
'lookalike finally suffering unwelcome' is the wordplay.
'lookalike' becomes 'twin' (twin can mean a lookalike).
'finally' suggests the final letters.
The last letters of 'suffering unwelcome' are 'ge'.
'twin'+'ge'='TWINGE'
(Other definitions for twinge that I've seen before include "Sharp localised pain" , "Sudden sharp pain" , "Sharp pang" , "A slight pain or pang" , "Sudden stab of pain" .)